The present invention relates to a color organic light emitting diode (OLED) display having improved lifetime and to a method of designing such a display.
Recent advances in flat panel display technology have included the advent of flat panel displays that are constructed using Organic Light Emitting Diodes. This flat panel display technology has many advantages over flat panel displays based on more popular liquid crystal display technology. Unlike liquid crystal displays that use the orientation of liquid crystal materials to modulate light from a large uniform backlight, OLED displays utilize materials that emit light. A full-color display is typically constructed by depositing three different materials that each emits a different color of light onto a substrate to create a full-color display, although they may also be constructed using a single emissive material and a color filter array. Depositing these separate light emissive elements in different spatial locations allows the construction of a full-color display when the light from the three primary colors are integrated by the human eye to allow the perception of a large gamut of colors from these few primary colors. During this deposition, equal areas of each of the three primary colors are commonly deposited onto the substrate.
Unfortunately, when equal area subpixels are used to construct a display using the OLED materials available today, the lifetime of the display is often limited by the lifetime of one of the OLED materials, typically blue. That is, when placed into a practical multicolor display, the time required for typical blue materials to deteriorate to produce half their original luminance is often only a fraction of the time required for typical green or red materials to deteriorate to the point that they produce half of their original luminance. For example, with one commonly available set of materials, the lifetime of the red emitting subpixels may be about 5.5 times as long as the lifetime of the blue emitting subpixels, and the lifetime of the green light emitting subpixels may be about 7 times as long as the lifetime of the blue light emitting subpixels, when each material set is driven at currents required to produce a standard color temperature display.
To maintain a well-balanced, full color display, it is important that the relative luminance of the three-colored materials be maintained throughout the lifetime of the display. If these relative luminance values change dramatically, images may have a serious color imbalance, and the user is likely to become dissatisfied with the display and consider the display to be at the end of its life. Some compensation can be made for the drop in blue efficiency over time by continually increasing the current density through the blue subpixels. However, it is known that increasing current densities tends to accelerate the luminance decay. Thus, the problem is actually worsened and the lifetime of the device before failure is shortened even more. Alternatively, one may reduce the luminance of the red and green to balance the blue, but this lowers the overall brightness of the display. Once the display becomes too dim, the user may also consider the display to be at the end of its useful lifetime, regardless of the relative luminance of the three colors. To maximize the useful lifetime of the display, it is important to maximize the time that the relative luminance of the three-color elements can be maintained while minimizing the loss of absolute luminance.
Flat panel displays with unequal areas of light emitting material have been discussed by Kim et al. in U.S. Patent Application 2002/0014837, published Feb. 7, 2002. Kim et al. discuss an OLED display in which the relative size of the red, green, and blue light emitting elements are adjusted based on the luminous efficiency of the color materials employed in an OLED display. It is commonly known that the available red OLED materials have significantly lower luminous efficiency than the existing green and blue OLED materials. Because of the lower efficiency of existing red OLED materials, if one wishes to maintain sub-pixels of equal size, the power per square area that must be provided to the low luminous efficiency material must be increased to obtain the desired light output. Using this criterion, Kim proposes an OLED display with twice as much red light emitting area as green and blue light emitting area. By creating displays with relatively larger areas of red emitting materials than green or blue materials, the relative power per square area can be somewhat equalized across the different colored materials. However, optimizing the display layout suggested by Kim et al., does not necessarily lead one to a design in which the lifetimes of the three materials are optimized.
U.S. Pat. No. 6,366,025 issued Apr. 2, 2002 to Yamada discloses an OLED display with unequal light emitting element areas, wherein the area of the light emitting elements are adjusted with the goal of improving the lifetime of the OLED display. Yamada considers the emission efficiency of the material, the chromaticity of each of the emissive materials and the chromaticity of the target display when attempting to determine the aim light emissive element areas. However, Yamada fails to discuss other important characteristics of OLED materials that will affect device lifetime, such as the differences in the inherent luminance stability over time of different materials. Yamada further does not consider important optical characteristics of the target display design, each of which will influence the overall lifetime of OLED materials.
There is a need therefore for an improved arrangement of the light emitting elements in an OLED display that compensates for the differences in the relative efficiencies and relative luminance stability over time of the different color emitting materials that are used to construct the display, thereby providing a display with a truly longer lifetime.
The need is met according to the present invention by providing an OLED color display device, in which a display pixel has a plurality of subpixels of different colors, wherein the areas of the subpixels are different in size based on the emission efficiency of the emissive elements, the chromaticity of a target display white point, and the relative luminance stability over time of the subpixels, thereby extending the useful lifetime of the display.
The present invention has the advantage of extending the useful lifetime of full-color OLED displays by taking into account the differences in the relative efficiencies and luminance stability over time of the materials employed in the displays.